1. Field of the Invention
The invention relates to self-adhesive addition-crosslinking silicone compositions, to addition-crosslinked silicone elastomers, and to processes for preparing them.
2. Background Art
It is known that the adhesion of addition-crosslinked silicone elastomers to numerous substrates such as plastics, metals or glasses, is low. If, for example, an addition-crosslinking silicone elastomer compound is applied to a substrate and subsequently crosslinked, the resultant silicone elastomer is generally removable from the substrate surface without problems, i.e., by application of low tensile forces. Indeed, in many cases, spontaneous detachment of the silicone elastomer from the substrate is observed. Since in numerous applications, however, a firm and lasting substrate adhesion of the silicone elastomer is of critical importance, a multiplicity of special measures have been proposed in order to obtain a firm bond between substrate and silicone elastomer.
In principle the adhesion of the assembly formed between silicone elastomer and substrate can be increased by altering the chemical and/or physical nature of the substrate or of the substrate surface in an appropriate way before the addition-crosslinking silicone elastomer composition is applied. This can be done, for example, by pretreating the substrate surface with adhesion promoter additives, commonly known as primers; plasma-treating the substrate surface; mixing special additives into the substrate; and/or purposively modifying the morphology of the substrate or increasing the surface roughness. Disadvantages of such measures include the need for additional process steps or for specific requirements to be imposed on the nature of the substrate.
The adhesion of the assembly formed between silicone elastomer and substrate can additionally be increased by purposively modifying the chemical and/or physical nature of the addition-crosslinking silicone elastomer compound. Numerous adhesion-promoting additives are known which, when admixed to the uncrosslinked silicone compound, are intended to allow self-adhesion of the resultant silicone elastomer to a variety of substrates. For example, European laid-open specification EP 0 875 536 A2 describes epoxy-functional alkoxysilane and/or alkoxysiloxane adhesion promoters. In U.S. Pat. No. 4,257,936, adducts of acryloyltrialkoxysilanes with cyclic hydropolysiloxanes act as adhesion promoters. U.S. Pat. No. 4,011,247 discloses epoxy adducts of hydropolysiloxanes, while U.S. Pat. No. 3,510,001 discloses alkoxysilane adducts of triallyl isocyanurate. U.S. Pat. No. 5,312,855 describes siloxane compounds having SiH and alkoxysilyl or glycidyl groups in combination with an organic compound having 2 or more allyl ester groups as adhesion promoters.
U.S. Pat. No. 5,438,094 discloses self-adhesive addition-crosslinking silicone compositions which comprise special organohydropolysiloxanes as crosslinkers and compounds having at least one aliphatic unsaturated group and two phenylene skeletons per molecule as adhesion promoters. The organohydrosiloxanes claimed are of the general formula (1), (2) or (3): R3Si(O—SiHR)nOSiR3 (1), (OSiHR)n cyclics (2), and R4-1Si(OSiHR2)1 (3). In the formulae (1), (2), and (3) the radicals R are substituted or unsubstituted monovalent hydrocarbon groups, 1 is 3 or 4, and n is an integer which is at least 3. Described as not inventive in patent U.S. Pat. No. 5,438,094, in contrast, is an organopolysiloxane component with the general formula (4): Me3Si—(SiHMe)5—(OSiMe2)8—SiMe3 (4).
The prior art self-adhesive addition-crosslinking silicone rubber compounds using the stated adhesion additives have in some cases improved adhesion to metals and some organic resins and plastics. A disadvantage, however, is that these self-adhesive addition-crosslinking silicone rubbers do not exhibit improved adhesion with respect, in particular, to problem plastics such as nylon resins, polycarbonate resins, and acrylic resins, for example.